MONTMORILLONITE UNDER HIGH H2O PRESSURES - STABILITY OF HYDRATE PHASES, REHYDRATION HYSTERESIS, AND THE EFFECT OF INTERLAYER CATIONS

Dehydration of Ca- and Mg-exchanged montmorillonite was studied along H2O isochores in the hydrothermal diamond-anvil cell by in situ X-ray diffraction using a synchrotron radiation source. At pressures between the H2O liquid-vapor (L-V) boundary and similar to 10 kbar, the dehyd ration temperature for the conversion from the 19 Angstrom hydration s tate to the 15 Angstrom hydration state occurred over the temperature range 260-350 degrees C for Ca-exchanged montmorillonite and 200-250 d egrees C for Mg-exchanged montmorillonite, with a slight increase with increasing pressure. For both materials, the rehydration from the 15 Angstrom to 19 Angstrom states occurred at the same temperature as deh ydration at pressures along the H2O L-V boundary, thus showing no hyst eresis. The rehydration hysteresis increased to nearly 75 degrees C at 6 kbar for Ca-exchanged montmorillonite and to the same amount at 2.5 kbar for Mg-exchanged montmorillonite. Dehydration experiments on Mg- exchanged montmorillonite along the isochores of 1.024 and 0.75 g/cm(3 ) showed conversion from the 15 Angstrom hydrate to the 12.5 Angstrom hydrate at 590-605 degrees C. The 12.5 Angstrom hydrate only partially rehydrated after cooling to room temperature along those two isochore s. In an experiment started from the 15 Angstrom state, in which the p ressure was below the H2O L-V curve, dehydration occurred at 400-500 d egrees C and rehydration at 430-350 degrees C. When our previous resul ts on Na-exchanged montmorillonite are combined with the current exper imental data, systematic trends can be found in the effect of pressure and interlayer-cation species on the dehydration temperature and rehy dration hysteresis.